Percent yield is expected to be less than 100% because it is:
Percent yield = [actual yield / theoretical yield] × 100
The theoretical yield is calcualted from the stoichiometry of the chemical equation, then the actual yield cannot be greater than the theoretical yield.
A theoretical yield greater than 100% is indication of an error.
Two errors can be thought:
- A wrong measurement of the amounts of reactants or products
- The presence of impurities in the reactants that lead to the actual mass is greater than the mass if the reactant is pure.